art rock vs rock and roll

art rock and rock and roll both are nouns.

Word	Noun	Adjective	Verb	Adverb
art rock	Yes	No	No	No
rock and roll	Yes	No	No	No

As nouns, rock and roll is a hypernym of art rock; that is, rock and roll is a word with a broader meaning than art rock:

art rock: a style of rock music that emerged in the 1970s; associated with attempts to combine rock with jazz and other forms; intended for listening and not dancing
rock and roll: a genre of popular music originating in the 1950s; a blend of black rhythm-and-blues with white country-and-western

Other hypernyms of art rock include rock-and-roll, rock'n'roll, rock 'n' roll, rock, rock music.

art rock (noun)	rock and roll (noun)
a style of rock music that emerged in the 1970s; associated with attempts to combine rock with jazz and other forms; intended for listening and not dancing	a genre of popular music originating in the 1950s; a blend of black rhythm-and-blues with white country-and-western